1. Field of the Invention
This invention relates generally to cable television communication systems. More particularly, the invention relates to a system which reduces noise ingress in the return communication path of a bi-directional cable television communication system.
2. Description of Related Art
Cable television (CATV) communication networks are being increasingly used for applications that require two-way transmission of data. Bi-directional CATV networks have become commonplace as the diversity of services has grown. Many CATV operators now provide networks which support interactive applications, including home banking and home shopping, or life-saving applications, such as medical alert and alarm services. CATV operators have even begun to challenge the regional bell operating companies (RBOCs) by offering telephone services over CATV networks.
Although bi-directional CATV networks have become commonplace, there are several problems which continue to plague CATV operators in offering interactive services. One of the major problems has been ingress noise, which is unwanted external signals entering the CATV network at weak points in the network such as shield discontinuities, faulty connectors and at a consumer's home through the settop terminal. Ingress noise presents an interfering signal on the CATV network, thereby degrading or even eliminating valid signals transmitted over the CATV network.
Ingress noise is particularly undesirable in bi-directional CATV systems since the bi-directional systems are significantly more susceptible to noise. In the upstream communication path, noise from every point in the CATV network is propagated, added and amplified in the upstream direction. As the signals travel toward the headend of the system each line in the CATV system is amplified and added together. It becomes extremely difficult at the headend, if not impossible, to detect ingress noise from valid signals sent from settop terminals.
One method of reducing ingress noise from settop terminals is to place a filter at each tap location corresponding to homes where no upstream communication equipment is present. This limits the ingress noise sources to tap locations corresponding to homes which utilize the upstream communication path. Since many of the services offered by CATV providers are interactive, nearly all homes utilize the upstream communication path. Therefore, this method of reducing ingress noise has diminishing benefits.
There have been attempts in the prior art to overcome the problems caused by unwanted ingress noise. For example, U.S. Pat. No. 5,235,619; and related U.S. Pat. Nos. 5,225,902; 5,155,590 and 5,142,690 disclose a bi-directional CATV communication system which compensates for unwanted ingress noise by communicating each message from the settop terminals to the headend over a plurality of frequencies and time slots. The headend receives and compares the redundant transmissions to confirm the transmitted message. This system requires high-speed hardware to be located both at the headend and at each settop terminal, resulting in a system which is inefficient and expensive.
Accordingly, there exists a need for a system for enhancing bi-directional CATV communication systems by reducing unwanted ingress noise in the upstream communication path.